This invention relates in general to crystal oscillators. More particularly, the invention relates to the temperature compensation of modulated crystal oscillator circuits.
In modern FM transmitter design practice, a direct frequency modulated crystal controlled oscillator is generally used as the primary frequency source. Typically, a varicap diode is employed as the modulating element. A bias is established for this diode and this bias is varied by application of a modulating signal superimposed on it. A typical representative circuit of this type is shown in FIG. 1 (PRIOR ART).
Referring to FIG. 1, the crystal (XTAL) controls the oscillator frequency and is operated near series resonance. The combination of crystal XTAL, inductor L1, varicap diode C1 and capacitors C2, C3 and C4 constitutes a resonant circuit of a Colpitts-type oscillator.
We denote the motional inductance of crystal XTAL as L.sub.s. The following relationship exists: ##EQU1## where .DELTA.X.sub.c1 =change in the reactance of the varicap and .DELTA.W is the change in the oscillator frequency resulting from a change in the varicap capacity C.sub.1.
To achieve adequate frequency stability over a wide temperature range, it is generally necessary to temperature compensate the oscillator circuit. Such temperature compensation is necessary because the resonant frequency of crystal XTAL is temperature dependent. Temperature compensation is usually accomplished by applying a temperature dependent voltage to varicap diode C1. Accurate temperature compensation is not particularly difficult for an individual oscillator which has known behavior if the bias of the varicap can be made appropriately temperature dependent. This is because the effect of the tolerances of LS and C1 can be readily absorbed. It is only necessary to determine the crystal resonant frequency as a function of temperature and then construct a circuit for applying the appropriate compensating temperature function to the DC bias applied to varicap diode C1.
Despite the fact that it is relatively simple to construct such a circuit for an individual oscillator, temperature compensation is not so simple when the oscillator is part of an overall circuit in which the crystal itself may be switched to change frequency. Under such circumstances, the solution to the temperature compensation problem becomes rather complex. A different amount of control is required due to the tolerance effect of L.sub.s and C1. The tolerance on L.sub.s is of the order of .+-.25%, that of the varicap diode C1 is .+-.15%. Furthermore, the motional inductance L.sub.s is frequency dependent. Thus, oscillators operating at different frequencies require different compensation voltages.
It would be highly desirable to be able to compensate for these tolerances without actually making measurements on the oscillator circuit or changing circuit elements. In the case of an oscillator that is to be frequency modulated, as in the case of an FM transmitter, the modulation level of control can be used to automatically provide the required temperature compensation for the oscillator by appropriately combining the modulation input from an audio circuit with a DC output of a temperature compensation circuit so that the composite signal can be applied to varicap diode C1 to produce both the proper modulation level and the required temperature compensation.
As a practical matter, such a scheme can be readily implemented. FCC regulations governing radio transmission over the airwaves require for certain radio services a modulation limiter circuit providing an accurately defined audio maximum signal level. This accurately defined audio maximim signal level provides a known transmitter frequency deviation, such as for example, 4 kHz. peak deviation. This deviation is typically set with a modulation potentiometer. If the proper amount of DC control input is coupled to the same modulation potentiometer, adjustment for proper frequency deviation will automatically produce the appropriate amount of temperature compensation regardless of component tolerances of the oscillator circuit. However, by combining these functions, there can be a certain degree of interaction between the frequency deviation adjustment and the center frequency setting. In other words, setting the deviation affects the center frequency. It is a cumbersome procedure to obtain the appropriate center frequency setting while at the same time achieving the desired deviation.